If cookies are enabled then: 1) we can read/write cookies 2) we can use sessions
If cookies are disabled: 1) we can't read/write cookies 2) we can't use sessions (cookie based) 3) we can use sessions by passing the SID in the query string
If you're asking "If we can use cookies then we can use cookies, but if we can't use cookies then we can't use cookies", then yes.
Also, note that the SID can be passed as a hidden form element as well. Other than that, I agree with Weedpacket.
horray for obvious answers! 🙂